A semiconductor light emitting diode (LED) has a long lifespan, permits a reduction in size and weight, exhibits strong orientation of light, and can be operated at low voltage. In addition, the semiconductor light emitting diode has high resistance against impact and vibration, does not require preheating and a complex drive circuit, and may be packaged in various shapes. Particularly, a nitride-based semiconductor light emitting diode has attracted much attention, since it has a high energy band gap to allow light output in a wide wavelength band from ultraviolet to blue/red colors and exhibits excellent physical/chemical stability to realize high efficiency and high output. Since such nitride semiconductor LEDs permit emission of white light through combination with existing red and green LEDs, it is expected that the nitride LED will replace existing incandescent lamps, fluorescent lamps, mercury lamps, and white lighting devices within several years.
However, conventional nitride-based LEDs are not satisfactory in terms of light output, luminous efficacy and price, and need improved performance. Particularly, the conventional LED needs improved light output, which is still lower than that of existing white light sources, and to overcome the problem of thermal stability caused by improving light output.
Meanwhile, a typical nitride semiconductor light emitting diode is manufactured by forming a nitride-based n-type layer, a nitride-based active layer and a nitride-based p-type layer on a sapphire substrate, and horizontally placing two electrodes for application of a power source to the n-type layer and the p-type layer. Since such a lateral type light emitting diode may be manufactured through a relatively simple process, it is advantageous in terms of low manufacturing cost, but the use of the sapphire substrate which is an insulator and exhibits low thermal conductivity leads to degradation in thermal stability upon high output and heat accumulation through application of electric current to large area.
To overcome such problems of the typical light emitting diode, a vertical type semiconductor light emitting diode and a flip chip type semiconductor light emitting diode are suggested. In such structure, a reflective layer is formed on a p-type electrode and allows light generated in an active layer to be emitted to the outside through an n-type electrode, and, instead of the sapphire substrate, a metal substrate having high thermal conductivity is used to allow application of electric current to large area and rapid heat dissipation, thereby securing high output and thermal stability. Since such a vertical type semiconductor light emitting diode achieves high output through an increase in maximum application current several times above that of the lateral type semiconductor light emitting diode, it is evaluated that the vertical type semiconductor light emitting diode can replace existing white lighting devices.
Meanwhile, in order to improve operating voltage characteristics of the vertical type semiconductor light emitting diode, the n-type electrode should have low resistance characteristics. For the vertical type semiconductor light emitting diode, a metal substrate or a semiconductor substrate such as Si, Ge, and the like is used as a support substrate and the sapphire substrate is removed via laser lift-off (LLO). At this time, it is difficult to perform annealing at high temperature after laser lift-off due to wafer bonding temperature and large difference in thermal expansion coefficients between the metal substrate and a GaN layer. Accordingly, Cr/Au and Ti/Al n-type ohmic electrodes which can be formed at room temperature without annealing are generally used in the art. However, such ohmic electrodes have a problem in that ohmic characteristics are easily deteriorated causing increase in operating voltage by heat generated during annealing for formation of a SiO2 protective layer after formation of the electrodes or during injection of high electric current in a large area light emitting diode. Further, the Ti/Al electrode has problems in that Al can be easily oxidized and the Ti/Al electrode can be easily etched by various kinds of solutions. Thus, there is a need for development of an n-type ohmic electrode which may exhibit low contact resistance immediately after deposition, while satisfying excellent thermal stability in order to maintain low contact resistance after thermal treatment.
Further, a semiconductor light emitting diode should have a large area in order to achieve further improved light output upon injection of high electric current. Here, the area of the electrode, for example, an n-type electrode, is also gradually increased in order to improve current spreading characteristics upon injection of high electric current. However, since a general n-type electrode such as a Cr/Au or Ti/Al electrode uses a thick Cr or Ti layer having low reflectivity, part of the n-type electrode absorbing light from the active layer increases with increasing area of the n-type electrode, causing obstruction against improvement of light output. Accordingly, there is also a need for an n-type ohmic electrode capable of exhibiting high reflectivity with low ohmic resistance.
Moreover, in fabrication of the vertical type semiconductor light emitting diode, a nitride semiconductor layer is first formed on a base substrate, a p-type electrode is formed on an upper surface of the nitride semiconductor layer, that is, on a gallium (Ga)-face, and an n-type electrode is formed on a lower surface of the nitride semiconductor layer, that is, on a nitrogen (N)-face, by separating the base substrate after attaching an assistant substrate to the p-type electrode. However, unlike the Ga-face, it is difficult to obtain good ohmic characteristics with the N-face without annealing and the difference in thermal expansion coefficients between the assistant substrate (metal substrate) and the nitride semiconductor layer makes it difficult to perform annealing itself. As such, a conventional Cr/Au or Ti/Al electrode formed on the N-face exhibits undesired ohmic characteristics and has low thermal stability.